Based on the dominance of rice-mediated pathway of CH4 transmission from the submerged soil to the atmosphere (Jia and Cai, 2003), there is evidence that genetic variations of rice and their effects on plant parameters influence CH4 emissions from irrigated rice systems (Su et al., 2015; Qin et al., 2015; Baruah et al., 2010). The present study also demonstrated a significant difference between rice genotypes for methane emissions, with commercial variety and the breeding line emitting significantly less methane than the Hybrid genotypes.
Previous studies have reported that the difference between rice genotypes on methane emissions is mainly related to rice phenotypic and physiological parameters, i.e. the number of plant tillers, plant above and belowground biomass (Sinha 1995; Setyanto et al., 2004; Khosa et al., 2010). Other mechanisms that may influence observed genotypic variations in CH4 emissions include differences in (1) root exudates, which represent methanogenic substrate (Kerdchoechuen, 2005); (2) the development of aerenchyma (Aulakh et al. 2000); and (3) the size of methane-oxidizing sites in the rhizosphere (Win et al., 2012; Gutierrez et al., 2014).
The roots of different rice genotypes may have other influences on the soil methanotrophic community composition (Lüke et al., 2011). Furthermore, some cultivars appear to allocate more of the products of photosynthesis to root exudation than others (Gutierrez et al., 2013; Su et al., 2015). Previous studies have reported that root exudates constitute an organic substrate for microbial organisms that could be utilized for methane production and oxidation by methanogens and methanotrophs, respectively (Win et al., 2012). Root exudates include simple sugars, which act as an electron donor under a flooded field, resulting in anaerobic conditions conducive to CH4 production (Wassman and Aulakh, 2000 and Le Mer and Roger, 2001).
To compare rice grain yield and methane emissions in irrigated rice systems, researchers have used yield-scaled emissions to indicate the global warming impact of rice production (Moiser et al., 2006; Pittelkow et al., 2013; Bayer et al., 2014). As suggested by Grassini and Cassman (2012), the yield-scaled metric is increasingly used to provide a measure of agronomic efficiency that begins to address both climate change and future food supply concerns. The present study aimed to identify rice genotypes with a high yield potential but lower methane emissions from among four rice genotypes. The results indicate that Hybrid 1 had high yield potential and moderate CH4 emissions resulting in low yield-scaled emissions. A higher rate of the partitioning of photosynthates to the developing panicles and grain accompanied by a higher rate of photosynthesis at the grain filling stage might be the reason for the higher grain yield in varieties with low methane emissions (Das and Baruah, 2008; Baruah et al., 2010). The partitioning of photosynthates to the panicles and grain will result in fewer carbohydrates being disposable for root exudates, an essential substrate for methanogens responsible for CH4 production (Su et al., 2015).
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Previously a global range of seasonal CH4 emissions was reported to be between 2.7 to 1059 kg ha− 1 (Minami, 1995; Yan et al., 2009). The magnitude of CH4 emissions observed in this study ranged between 43.4–70.7 kg CH4 ha− 1, well within the global range. Within the LAC region, using the commercial variety El Paso 144, a study conducted in Uruguay reported cumulative emissions ranging between 172.0 to 207.0 kg CH4 ha− 1 (Irisarri et al., 2012). Another study conducted in Uruguay, using the same commercial variety El Paso 144, also reported higher cumulative emissions ranging from 208.0 to 249.0 kg CH4 ha− 1 (Tarlera et al., 2016). In Colombia, a study conducted using the commercial variety FEDEARROZ-60 reported lower cumulative emissions of 7.5 and 19.5 kg CH4 ha− 1 in 2015 and 2016, respectively (Chirinda et al., 2017). In Brazil, CH4 cumulative emissions observed for variety IRGA, 424 were 303.0-424.0 kg CH4 ha− 1 (Zschornack et al., 2016) which was higher than those reported in the present study. The varietal differences may have contributed to the wide variation in the LAC region's methane emissions. The ideal rice cultivars for reducing methane emissions would probably need a high harvesting index, fewer ineffective tillers, panicles, and nodes (Wang et al., 1997). Additionally, the selection or breeding of rice genotypes that do not have well-developed aerenchyma systems may also mitigate CH4 emissions (Wassmann et al., 1993; Kludze et al., 1993). Focusing on rice cultivars for mitigating CH4 emissions is a more manageable approach, as it does not require farmers to change agronomic practices significantly (Balakrishnan et al., 2018).
Perspectives
While there is a need for further studies that explore CH4 emissions from more rice varieties and hybrids, our findings suggest a breeding solution for mitigating CH4 emissions from irrigated rice systems. Climate change mitigation-relevant breeding may need to focus on root systems – the interface between the plant (through which CH4 is transported to the atmosphere) and the soil (where the methane is formed). Specifically, our findings propose that breeding for shorter roots, lower root volume, and surface area without compromising yields may be beneficial in reducing rice-based methane emissions. On the other hand, to further incentivize the adoption of high-yielding and low CH4 emitting rice, such rice should be considered a clean development technology that could qualify for carbon credits. A low-yielding rice variety may result in low absolute emissions (per unit area). However, farmers will require more land to produce sufficient rice to reduce rice deficits in the region. Promoting hybrids could be a more promising approach for simultaneously achieving food security and emission goals.